What they do: With its signature Spartan interface, Craigslist serves up local classifieds and forums covering goods, services, jobs and relationships in 450 cities worldwide.

Disruptive qualities: Craigslist gets 15 million page views every month, putting them comfortably in the top 10 English-language sites amid the Googles, Yahoos and eBays of the world. That’s not just a staggering number of eyeballs, it’s also a death sentence for local media—especially newspapers—that survive on the money generated by classified advertising.

The tech that makes them tick: Simple Web interface, which is served up by collocation services from San Francisco’s 365 Main, keeps Craigslist quick, with redundant systems readily available, and, as always, it’s free to most end users. Not even Google, which was expected to crush Craigslist with its Google Base service two years ago, can come close to the traction and market penetration of Craig Newmark’s brainchild.

Who they are disrupting: Local daily and weekly newspapers and other traditional media are feeling the pain.

Getting flagged? Craigslist has always generated controversy for its reluctance to censor its users. The site is frequently cited in court cases involving local prostitution rings, and a Michigan woman was recently arrested for placing a Craigslist ad seeking a contract killer to take out a romantic rival.